I read that the electronic health record (not electronic medical record) stores the complete history of a patient in the US. So, do all physicians use the same EHR system? (Like facebook i.e. there is 1 facebook for everyone) or does each hospital/doctor have their own EHR and they have to transfer/request patient data from the patient's previous doctor?
Short answer: there are many different EHR systems, and they don’t like to talk to each other.
Long answer: There are a lot of different pieces of software out there for EHR systems. Soarian, Epic, Powerchart are a few of the ones I have worked with. Usually, an EHR system will be throughout a Health System. All the hospitals in that system will be able to share data and medical records. To send medical records to a physician or hospital not in that system, they, in my experience, are printed and faxed over.
Even if two systems use the same EHR program, those programs are usually customized to that system, and won’t necessarily transfer data between them.
I highly recommend requesting and keeping your own copies of your medical records for this very reason.